Top layer:               pcbname.GTL 
Bottom layer:            pcbname.GBL 
Solder Stop Mask top:    pcbname.GTS 
Solder Stop Mask Bottom  pcbname.GBS 
Silk Top:                pcbname.GTO 
Silk Bottom              pcbname.GBO 
NC Drill:                pcbname.DRL


extra files: 

SMD solder paste top:    pcbname.SMT
SMD solder paste buttom: pcbname.SMB 



FreeduinoMini1.lay is a Sprint-Layout5 file. From http://www.abacom-online.de/uk/html/sprint-layout.html



You may view the Gerber files using the GerbV open source program

Project Home page with links for *nix and Mac files 
http://gerbv.gpleda.org/index.html

Sourceforge with *nix and Windows files 
http://sourceforge.net/projects/gerbv/files/gerbv/gerbv-2.5.0/

